This invention relates to an apparatus for the production of foamed material from a reaction mixture of free-flowing reaction components which contains a dissolved foaming agent, consisting of a mixing chamber having an outlet opening and having feed lines for the reaction components, wherein at least one of the feed lines comprises an element for the introduction and dissolution of foaming agent, and wherein a depressurisation element (creamer) adjoins the outlet opening (DE 44 22 568 C1).
During the production of foamed material, and during the production of polyurethane foamed material from isocyanate and polyol in particular, the function of the dissolved foaming agent is to change into its gaseous state due to depressurization on its emergence from the creamer, to become attached to the nucleating seeds formed, and to form pores in the foamed material.
Suitable foaming agents include fluorinated hydrocarbons, which can no longer be used for reasons of protecting the environment, pentane, and carbon dioxide, which has been used recently in particular.
In order to produce a foamed material with a uniform apparent density, it is necessary to ensure firstly that sufficient, uniformly distributed nucleating seeds are formed before the depressurization of the reaction mixture and secondly that the entire foaming agent is not depressurized until it has left the creamer, and when it changes to its gaseous state and becomes attached to nucleating seeds which have already been formed, pores of uniform size are formed which are uniformly distributed in the foamed material.
In the creamer, which is disposed downstream of the mixing chamber, the flow of reaction mixture is therefore throttled by means of at least one perforated plate or sieve plate which serves as a throttle element and which thereby ensures that the requisite pressure is maintained so that the foaming agent remains in its dissolved state. 10% of this at least one sieve plate is open, for example, and it comprises about 1270 holes per cm2 with a diameter of 0.1 mm. Very finely perforated plates or fine sieve plates are used, wherein the number of holes per cm2 and the number of sieve plates depend, amongst other factors, on the pressure which has to be maintained.
In order to obtain the optimum foamed material, it has already been proposed that the size of the active surface area of the perforated plate be adjusted correspondingly at the commencement of the foaming process by means of screens which can be adjusted outside the creamer and which are thus held on one side (DE 195 24 434 A1). However, since these screens are disposed on an inclined holding device, they are unstable; and since areas of different size of the perforated plates or sieve plates are covered depending on the adjustment made, the flow is inhomogeneous on account of the varying cross-section of the flow channel or on account of the varying flow path.
Attempts have also already been made to make the adjustment by means of a gap with an adjustable height (DE 195 24 434 A1). This height-adjustable gap is disposed between two solid, flared faces or between a solid face and a perforated plate or sieve plate. It has been shown here also that adjusting the gap height not only changes the pressure in the desired manner, but also changes the flow relationships in an unwanted manner. However, the formation of nucleating seeds is also changed thereby in a disadvantageous manner, and uniform formation of pores cannot be ensured.
Finally, it has already been proposed that a calming element in the form of a calming sieve be associated with a flared gap (EP 0719 627 A2). Here also, homogeneous flow cannot be ensured, because the efflux of reaction mixture from the gap occurs substantially parallel to the calming sieve, due to which there is a broad spectrum of residence times and a deflection at right angles has to be made again for passage through the calming sieve, whereby the resulting swirl also has an undesirable sustaining effect on the width of the residence time spectrum.
Carbon dioxide is currently used in particular as a foaming agent for the production of polyurethane foamed material, even though this technique is difficult to control. This applies both to foaming in situ and to continuous production processes in particular, such as the production of block foamed material, the production of slabs on a double conveyor belt, and the coating of textiles. Since carbon dioxide suddenly changes into its gaseous state on depressurization, the susceptibility to problems is particularly high here, and it is difficult to achieve perfect quality of the foamed material.
If the installation is designed for constant production conditions, foamed material of satisfactory quality is generally obtained. Constant production conditions over extended periods are rare, however, on account of high throughputs and very different customer requirements.
During the continuous production of polyurethane foamed material, for example, it is necessary to produce different types of foamed materials with different apparent densities. In order to achieve this, the proportion of foaming agent has to be varied. Accordingly, the creamer has to be matched to the new production conditions by the installation of further sieve plates or perforated plates or by the replacement of sieve plates or perforated plates disposed in the flow channel by others with a different passageway area or with a different permeability. This of course has the disadvantage that production has to be stopped for a certain time for the replacement of the sieve plates or perforated plates. When it is considered that modern block foamed material installations operate at a production speed of about 5 m/minute, the high production outage is obvious. Furthermore, when the block foamed material installation is re-started the first few meters of block foam which are produced are rejects. Moreover, it is not possible to make fine adjustments by means of fixedly installed perforated plates or sieve plates in order to optimize the processing of the reaction mixture at the commencement of production.
During foaming in situ, which is of course operated with short interruptions during operation on account of the cyclic production procedure, short-term changes in production conditions likewise have to be made frequently. When the production conditions are changed, the creamer has to be flushed out if necessary.
A change in production conditions generally occurs due to a change in the proportion of foaming agent in the reaction mixture, due to a change in throughput, and/or due to a change in the ratio by volume of the reaction components to one another.
Not only is the aforementioned fine adjustment often necessary at the commencement of production in order to obtain the optimum foamed materials, but subsequent adjustment during operation is often desirable also.
The object of the present invention was therefore to create an improved apparatus for producing the optimum foamed materials with a uniform pore size or bubble size from reaction mixtures containing dissolved foaming agent, which installation can be adjusted during production or can be adjusted to match changed production conditions.
This object is achieved by the provision of two perforated plates or sieve plates, which are constructed as throttle plates and which are disposed transversely to the direction of flow in said depressurisation element (creamer), wherein the spacing a between said two throttle plates can be varied during operation, and by the provision of a calming element which is disposed after the second throttle plate in the direction of flow.